Beast Friendship and swarms

Given that a swarm is handled as a single creature, would it be possible for a character with Beast Friendship to take said swarm as a single henchman?

Only if he wears it

No. It's treated as a single creature for simplicity / mechanics, but it is not actually a single creature.

That said, the legend of the Pied Piper certainly makes this arguable, so if you want to run with it for your campaign, go for it!